//===- ConstantHoisting.cpp - Prepare code for expensive constants --------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass identifies expensive constants to hoist and coalesces them to // better prepare it for SelectionDAG-based code generation. This works around // the limitations of the basic-block-at-a-time approach. // // First it scans all instructions for integer constants and calculates its // cost. If the constant can be folded into the instruction (the cost is // TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't // consider it expensive and leave it alone. This is the default behavior and // the default implementation of getIntImmCost will always return TCC_Free. // // If the cost is more than TCC_BASIC, then the integer constant can't be folded // into the instruction and it might be beneficial to hoist the constant. // Similar constants are coalesced to reduce register pressure and // materialization code. // // When a constant is hoisted, it is also hidden behind a bitcast to force it to // be live-out of the basic block. Otherwise the constant would be just // duplicated and each basic block would have its own copy in the SelectionDAG. // The SelectionDAG recognizes such constants as opaque and doesn't perform // certain transformations on them, which would create a new expensive constant. // // This optimization is only applied to integer constants in instructions and // simple (this means not nested) constant cast expressions. For example: // %0 = load i64* inttoptr (i64 big_constant to i64*) //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/ADT/SmallSet.h" #include "llvm/ADT/SmallVector.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/TargetTransformInfo.h" #include "llvm/IR/Constants.h" #include "llvm/IR/Dominators.h" #include "llvm/IR/IntrinsicInst.h" #include "llvm/Pass.h" #include "llvm/Support/Debug.h" #include <tuple> using namespace llvm; #define DEBUG_TYPE "consthoist" STATISTIC(NumConstantsHoisted, "Number of constants hoisted"); STATISTIC(NumConstantsRebased, "Number of constants rebased"); namespace { struct ConstantUser; struct RebasedConstantInfo; typedef SmallVector<ConstantUser, 8> ConstantUseListType; typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType; /// \brief Keeps track of the user of a constant and the operand index where the /// constant is used. struct ConstantUser { Instruction *Inst; unsigned OpndIdx; ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) { } }; /// \brief Keeps track of a constant candidate and its uses. struct ConstantCandidate { ConstantUseListType Uses; ConstantInt *ConstInt; unsigned CumulativeCost; ConstantCandidate(ConstantInt *ConstInt) : ConstInt(ConstInt), CumulativeCost(0) { } /// \brief Add the user to the use list and update the cost. void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) { CumulativeCost += Cost; Uses.push_back(ConstantUser(Inst, Idx)); } }; /// \brief This represents a constant that has been rebased with respect to a /// base constant. The difference to the base constant is recorded in Offset. struct RebasedConstantInfo { ConstantUseListType Uses; Constant *Offset; RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset) : Uses(Uses), Offset(Offset) { } }; /// \brief A base constant and all its rebased constants. struct ConstantInfo { ConstantInt *BaseConstant; RebasedConstantListType RebasedConstants; }; /// \brief The constant hoisting pass. class ConstantHoisting : public FunctionPass { typedef DenseMap<ConstantInt *, unsigned> ConstCandMapType; typedef std::vector<ConstantCandidate> ConstCandVecType; const TargetTransformInfo *TTI; DominatorTree *DT; BasicBlock *Entry; /// Keeps track of constant candidates found in the function. ConstCandVecType ConstCandVec; /// Keep track of cast instructions we already cloned. SmallDenseMap<Instruction *, Instruction *> ClonedCastMap; /// These are the final constants we decided to hoist. SmallVector<ConstantInfo, 8> ConstantVec; public: static char ID; // Pass identification, replacement for typeid ConstantHoisting() : FunctionPass(ID), TTI(nullptr), DT(nullptr), Entry(nullptr) { initializeConstantHoistingPass(*PassRegistry::getPassRegistry()); } bool runOnFunction(Function &Fn) override; const char *getPassName() const override { return "Constant Hoisting"; } void getAnalysisUsage(AnalysisUsage &AU) const override { AU.setPreservesCFG(); AU.addRequired<DominatorTreeWrapperPass>(); AU.addRequired<TargetTransformInfo>(); } private: /// \brief Initialize the pass. void setup(Function &Fn) { DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); TTI = &getAnalysis<TargetTransformInfo>(); Entry = &Fn.getEntryBlock(); } /// \brief Cleanup. void cleanup() { ConstantVec.clear(); ClonedCastMap.clear(); ConstCandVec.clear(); TTI = nullptr; DT = nullptr; Entry = nullptr; } Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const; Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const; void collectConstantCandidates(ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx, ConstantInt *ConstInt); void collectConstantCandidates(ConstCandMapType &ConstCandMap, Instruction *Inst); void collectConstantCandidates(Function &Fn); void findAndMakeBaseConstant(ConstCandVecType::iterator S, ConstCandVecType::iterator E); void findBaseConstants(); void emitBaseConstants(Instruction *Base, Constant *Offset, const ConstantUser &ConstUser); bool emitBaseConstants(); void deleteDeadCastInst() const; bool optimizeConstants(Function &Fn); }; } char ConstantHoisting::ID = 0; INITIALIZE_PASS_BEGIN(ConstantHoisting, "consthoist", "Constant Hoisting", false, false) INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) INITIALIZE_AG_DEPENDENCY(TargetTransformInfo) INITIALIZE_PASS_END(ConstantHoisting, "consthoist", "Constant Hoisting", false, false) FunctionPass *llvm::createConstantHoistingPass() { return new ConstantHoisting(); } /// \brief Perform the constant hoisting optimization for the given function. bool ConstantHoisting::runOnFunction(Function &Fn) { DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n"); DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n'); setup(Fn); bool MadeChange = optimizeConstants(Fn); if (MadeChange) { DEBUG(dbgs() << "********** Function after Constant Hoisting: " << Fn.getName() << '\n'); DEBUG(dbgs() << Fn); } DEBUG(dbgs() << "********** End Constant Hoisting **********\n"); cleanup(); return MadeChange; } /// \brief Find the constant materialization insertion point. Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst, unsigned Idx) const { // If the operand is a cast instruction, then we have to materialize the // constant before the cast instruction. if (Idx != ~0U) { Value *Opnd = Inst->getOperand(Idx); if (auto CastInst = dyn_cast<Instruction>(Opnd)) if (CastInst->isCast()) return CastInst; } // The simple and common case. This also includes constant expressions. if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst)) return Inst; // We can't insert directly before a phi node or landing pad. Insert before // the terminator of the incoming or dominating block. assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!"); if (Idx != ~0U && isa<PHINode>(Inst)) return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator(); BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock(); return IDom->getTerminator(); } /// \brief Find an insertion point that dominates all uses. Instruction *ConstantHoisting:: findConstantInsertionPoint(const ConstantInfo &ConstInfo) const { assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry."); // Collect all basic blocks. SmallPtrSet<BasicBlock *, 8> BBs; for (auto const &RCI : ConstInfo.RebasedConstants) for (auto const &U : RCI.Uses) BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent()); if (BBs.count(Entry)) return &Entry->front(); while (BBs.size() >= 2) { BasicBlock *BB, *BB1, *BB2; BB1 = *BBs.begin(); BB2 = *std::next(BBs.begin()); BB = DT->findNearestCommonDominator(BB1, BB2); if (BB == Entry) return &Entry->front(); BBs.erase(BB1); BBs.erase(BB2); BBs.insert(BB); } assert((BBs.size() == 1) && "Expected only one element."); Instruction &FirstInst = (*BBs.begin())->front(); return findMatInsertPt(&FirstInst); } /// \brief Record constant integer ConstInt for instruction Inst at operand /// index Idx. /// /// The operand at index Idx is not necessarily the constant integer itself. It /// could also be a cast instruction or a constant expression that uses the // constant integer. void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx, ConstantInt *ConstInt) { unsigned Cost; // Ask the target about the cost of materializing the constant for the given // instruction and operand index. if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst)) Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx, ConstInt->getValue(), ConstInt->getType()); else Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(), ConstInt->getType()); // Ignore cheap integer constants. if (Cost > TargetTransformInfo::TCC_Basic) { ConstCandMapType::iterator Itr; bool Inserted; std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0)); if (Inserted) { ConstCandVec.push_back(ConstantCandidate(ConstInt)); Itr->second = ConstCandVec.size() - 1; } ConstCandVec[Itr->second].addUser(Inst, Idx, Cost); DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) dbgs() << "Collect constant " << *ConstInt << " from " << *Inst << " with cost " << Cost << '\n'; else dbgs() << "Collect constant " << *ConstInt << " indirectly from " << *Inst << " via " << *Inst->getOperand(Idx) << " with cost " << Cost << '\n'; ); } } /// \brief Scan the instruction for expensive integer constants and record them /// in the constant candidate vector. void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap, Instruction *Inst) { // Skip all cast instructions. They are visited indirectly later on. if (Inst->isCast()) return; // Can't handle inline asm. Skip it. if (auto Call = dyn_cast<CallInst>(Inst)) if (isa<InlineAsm>(Call->getCalledValue())) return; // Scan all operands. for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) { Value *Opnd = Inst->getOperand(Idx); // Visit constant integers. if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) { collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); continue; } // Visit cast instructions that have constant integers. if (auto CastInst = dyn_cast<Instruction>(Opnd)) { // Only visit cast instructions, which have been skipped. All other // instructions should have already been visited. if (!CastInst->isCast()) continue; if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) { // Pretend the constant is directly used by the instruction and ignore // the cast instruction. collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); continue; } } // Visit constant expressions that have constant integers. if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { // Only visit constant cast expressions. if (!ConstExpr->isCast()) continue; if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) { // Pretend the constant is directly used by the instruction and ignore // the constant expression. collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt); continue; } } } // end of for all operands } /// \brief Collect all integer constants in the function that cannot be folded /// into an instruction itself. void ConstantHoisting::collectConstantCandidates(Function &Fn) { ConstCandMapType ConstCandMap; for (Function::iterator BB : Fn) for (BasicBlock::iterator Inst : *BB) collectConstantCandidates(ConstCandMap, Inst); } /// \brief Find the base constant within the given range and rebase all other /// constants with respect to the base constant. void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S, ConstCandVecType::iterator E) { auto MaxCostItr = S; unsigned NumUses = 0; // Use the constant that has the maximum cost as base constant. for (auto ConstCand = S; ConstCand != E; ++ConstCand) { NumUses += ConstCand->Uses.size(); if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost) MaxCostItr = ConstCand; } // Don't hoist constants that have only one use. if (NumUses <= 1) return; ConstantInfo ConstInfo; ConstInfo.BaseConstant = MaxCostItr->ConstInt; Type *Ty = ConstInfo.BaseConstant->getType(); // Rebase the constants with respect to the base constant. for (auto ConstCand = S; ConstCand != E; ++ConstCand) { APInt Diff = ConstCand->ConstInt->getValue() - ConstInfo.BaseConstant->getValue(); Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff); ConstInfo.RebasedConstants.push_back( RebasedConstantInfo(std::move(ConstCand->Uses), Offset)); } ConstantVec.push_back(ConstInfo); } /// \brief Finds and combines constant candidates that can be easily /// rematerialized with an add from a common base constant. void ConstantHoisting::findBaseConstants() { // Sort the constants by value and type. This invalidates the mapping! std::sort(ConstCandVec.begin(), ConstCandVec.end(), [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) { if (LHS.ConstInt->getType() != RHS.ConstInt->getType()) return LHS.ConstInt->getType()->getBitWidth() < RHS.ConstInt->getType()->getBitWidth(); return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue()); }); // Simple linear scan through the sorted constant candidate vector for viable // merge candidates. auto MinValItr = ConstCandVec.begin(); for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end(); CC != E; ++CC) { if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) { // Check if the constant is in range of an add with immediate. APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue(); if ((Diff.getBitWidth() <= 64) && TTI->isLegalAddImmediate(Diff.getSExtValue())) continue; } // We either have now a different constant type or the constant is not in // range of an add with immediate anymore. findAndMakeBaseConstant(MinValItr, CC); // Start a new base constant search. MinValItr = CC; } // Finalize the last base constant search. findAndMakeBaseConstant(MinValItr, ConstCandVec.end()); } /// \brief Updates the operand at Idx in instruction Inst with the result of /// instruction Mat. If the instruction is a PHI node then special /// handling for duplicate values form the same incomming basic block is /// required. /// \return The update will always succeed, but the return value indicated if /// Mat was used for the update or not. static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) { if (auto PHI = dyn_cast<PHINode>(Inst)) { // Check if any previous operand of the PHI node has the same incoming basic // block. This is a very odd case that happens when the incoming basic block // has a switch statement. In this case use the same value as the previous // operand(s), otherwise we will fail verification due to different values. // The values are actually the same, but the variable names are different // and the verifier doesn't like that. BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx); for (unsigned i = 0; i < Idx; ++i) { if (PHI->getIncomingBlock(i) == IncomingBB) { Value *IncomingVal = PHI->getIncomingValue(i); Inst->setOperand(Idx, IncomingVal); return false; } } } Inst->setOperand(Idx, Mat); return true; } /// \brief Emit materialization code for all rebased constants and update their /// users. void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset, const ConstantUser &ConstUser) { Instruction *Mat = Base; if (Offset) { Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst, ConstUser.OpndIdx); Mat = BinaryOperator::Create(Instruction::Add, Base, Offset, "const_mat", InsertionPt); DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0) << " + " << *Offset << ") in BB " << Mat->getParent()->getName() << '\n' << *Mat << '\n'); Mat->setDebugLoc(ConstUser.Inst->getDebugLoc()); } Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx); // Visit constant integer. if (isa<ConstantInt>(Opnd)) { DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset) Mat->eraseFromParent(); DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); return; } // Visit cast instruction. if (auto CastInst = dyn_cast<Instruction>(Opnd)) { assert(CastInst->isCast() && "Expected an cast instruction!"); // Check if we already have visited this cast instruction before to avoid // unnecessary cloning. Instruction *&ClonedCastInst = ClonedCastMap[CastInst]; if (!ClonedCastInst) { ClonedCastInst = CastInst->clone(); ClonedCastInst->setOperand(0, Mat); ClonedCastInst->insertAfter(CastInst); // Use the same debug location as the original cast instruction. ClonedCastInst->setDebugLoc(CastInst->getDebugLoc()); DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n' << "To : " << *ClonedCastInst << '\n'); } DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst); DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); return; } // Visit constant expression. if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) { Instruction *ConstExprInst = ConstExpr->getAsInstruction(); ConstExprInst->setOperand(0, Mat); ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst, ConstUser.OpndIdx)); // Use the same debug location as the instruction we are about to update. ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc()); DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n' << "From : " << *ConstExpr << '\n'); DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n'); if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) { ConstExprInst->eraseFromParent(); if (Offset) Mat->eraseFromParent(); } DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n'); return; } } /// \brief Hoist and hide the base constant behind a bitcast and emit /// materialization code for derived constants. bool ConstantHoisting::emitBaseConstants() { bool MadeChange = false; for (auto const &ConstInfo : ConstantVec) { // Hoist and hide the base constant behind a bitcast. Instruction *IP = findConstantInsertionPoint(ConstInfo); IntegerType *Ty = ConstInfo.BaseConstant->getType(); Instruction *Base = new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP); DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB " << IP->getParent()->getName() << '\n' << *Base << '\n'); NumConstantsHoisted++; // Emit materialization code for all rebased constants. for (auto const &RCI : ConstInfo.RebasedConstants) { NumConstantsRebased++; for (auto const &U : RCI.Uses) emitBaseConstants(Base, RCI.Offset, U); } // Use the same debug location as the last user of the constant. assert(!Base->use_empty() && "The use list is empty!?"); assert(isa<Instruction>(Base->user_back()) && "All uses should be instructions."); Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc()); // Correct for base constant, which we counted above too. NumConstantsRebased--; MadeChange = true; } return MadeChange; } /// \brief Check all cast instructions we made a copy of and remove them if they /// have no more users. void ConstantHoisting::deleteDeadCastInst() const { for (auto const &I : ClonedCastMap) if (I.first->use_empty()) I.first->eraseFromParent(); } /// \brief Optimize expensive integer constants in the given function. bool ConstantHoisting::optimizeConstants(Function &Fn) { // Collect all constant candidates. collectConstantCandidates(Fn); // There are no constant candidates to worry about. if (ConstCandVec.empty()) return false; // Combine constants that can be easily materialized with an add from a common // base constant. findBaseConstants(); // There are no constants to emit. if (ConstantVec.empty()) return false; // Finally hoist the base constant and emit materialization code for dependent // constants. bool MadeChange = emitBaseConstants(); // Cleanup dead instructions. deleteDeadCastInst(); return MadeChange; }